[SelectionDAG] Add computeKnownBits support for ISD::STEP_VECTOR (#80452)

This handles two cases where we can work out some known-zero bits for
ISD::STEP_VECTOR.

The first case handles when we know the low bits are zero because the
step
amount is a power of two. This is taken from
https://reviews.llvm.org/D128159

,
and even though the original patch didn't end up landing this case due
to it
not having any test difference, I've included it here for completeness's
sake.

The second case handles the case when we have an upper bound on
vscale_range.
We can use this to work out the upper bound on the number of elements,
and thus
what the maximum step will be. From the maximum step we then know which
hi bits
are zero.

On its own, computing the known hi bits results in some small
improvements for
RVV with -mrvv-vector-bits=zvl across the llvm-test-suite. However I'm
hoping
to be able to use this later to reduce the LMUL in index calculations
for
vrgather/indexed accesses.

---------

Co-authored-by: Philip Reames <preames@rivosinc.com>